1. Field of the Invention
The present invention relates to a biomedical device designed for providing stimulation to initiate neuromuscular recovery following peripheral nerve injury or surgical repair.
Especially, the present invention relates to an electrical stimulator system, that is implantable within a body, for applying electrical stimulation to initiate neuromuscular recovery at an affected area in the body.
2. General Background
The central nervous system is connected to the muscles, skin and other organs of the body by peripheral nerves from the spinal cord and brainstem to various peripheral structures and organs. Most such nerves arise from a nerve plexus, where sensory and motor axons bound for various destinations become highly intermingled.
The sensory axons arise from primary sensory cell bodies located in or near the central nervous system. They extend outward to innervate various sensory receptor structures in skin and muscles, where they are excited to produce electrical impulses that are conducted back along the sensory axon and into the central nervous system. The motor axons arise from the cell bodies of motoneurons that are located in the central nervous system and that extend into muscles where they make synaptic connections with muscle fibers.
When the central nervous system makes a muscle contract, it does so by sending electrical impulses to those neuromuscular synapses at the ends of the motor axons of the motoneurons. This results in the release of a chemical transmitter, which in turn induces an electrical signal in the muscle fibers. This electrical signal acts through other chemical pathways within the muscle fiber to result in generation of contractile force.
When a peripheral nerve or nerve plexus is cut, crushed, or damaged, electrical impulses cease to pass through both the sensory and motor axons at the site of injury. In the part of the peripheral nerve that is distal to the injury, both the sensory and motor axons may be cut off from their cell bodies, which normally provide essential chemicals required to maintain the axons. Thus, these axons degenerate and die within a few days of injury, a process referred to as “Wallerian degeneration.” In the proximal stump of the injured peripheral nerve, the damaged axons typically seal themselves off and undergo a complex sequence of changes in preparation for regrowth back toward the sensory and motor structures that they originally innervated.
Throughout the peripheral nerves, the larger axons (responsible for motor functions and important sensory functions) are surrounded by sheaths composed of specialized supporting cells called Schwann cells. When the distal portion of an injured axon dies, these Schwann cell sheaths remain intact indefinitely.
Schwann cell sheaths surrounding sensory and motor axons appear to have different properties. They undergo various chemical changes that may play an important role in facilitating the regeneration of the damaged sensory and motor axons by supplying guidance mechanisms and trophic factors. The regrowing axons must locate and then grow into and through the correct type of Schwann cell sheaths, which direct them toward denervated skin, muscles and any other organs to which the peripheral nerve should be connected.
Unfortunately, many of the motor axons that normally convey electrical commands to muscle fibers start to grow into the open ends of Schwann cell sheaths that once surrounded sensory axons. These sensory sheaths direct the regenerating motor axons toward the many sensory receptors in muscle and skin. Conversely, many of the regenerating sensory axons enter sheaths that direct them toward muscle fibers, with which they cannot make effective synapses. These inappropriate connections are not useful in reestablishing function.
In addition, regrowth of peripheral nerve axons occurs slowly from the site of injury (roughly 2-3 millimeters a day). During the time required for regenerating motor axons to reach denervated muscle fibers, the denervated muscle fibers are not receiving any chemical or electrical activation. Muscle fibers that are not activated occasionally will atrophy, becoming small and weak and sometimes disappearing entirely, to be replaced by fat and connective tissue. If the peripheral nerve injury is far from the denervated muscles or if the regenerative process is delayed, the muscle fibers may be so atrophic by the time they are reinnervated that recovery of motor function will be slow and incomplete.
Many of the processes that govern both the normal function of muscles and nerves and their recovery from a denervation injury are believed to be influenced by electrical activity of the nerves. Recently, it has been reported that electrical stimulation of the proximal stump of a cut nerve shortly after it has been surgically repaired reduces the number of motor axons that wind up regenerating into cutaneous sensory nerve branches. The Schwann cell sheaths surrounding sensory versus motor axons appear to react differently during the processes of denervation and reinnervation, with motor sheaths increasing the production of biochemical substances, such as the carbohydrate epitope named L2/HNK-1. Cell bodies of motoneurons are also affected by the backward or antidromic excitation of cell bodies by electrical stimulation. Stimulated cells increase their genetic expression of signaling and trophic molecules such as BDNF and its receptor trkB.
Electrical stimulation appears to augment difference in the way that motor and sensory axons recover, which contributes to the accuracy of sensory and motor reinnervation. Other studies have claimed that the application of weak negative electrical fields along the axis of nerve regeneration or pulsed electromagnetic fields accelerates the rate of outgrowth of the regenerating axons. Electrical activation of the regenerating axons may also accelerate their rate of maturation, i.e. enlargement of axon caliber, remyelination and increase in conduction velocity. Still other studies indicate that the process of recovery from muscle atrophy can be augmented by using electrical stimulation to exercise atrophic muscle fibers once their efferent neural connections have been reestablished.
However, it is difficult to stimulate peripheral nerves, particularly main nerves and nerve plexuses located deep within the proximal limbs or trunk. Although some methods have been suggested, each has at least some of the following important disadvantages:
(i) Electrical stimulation can be applied to the surface of the skin, but the intensity must be very high to influence the target nerves. This intensity is likely to produce uncomfortable sensations from stimulation of intact skin nerves and even irritation or damage of the underlying skin. It is also difficult to determine the effective level of stimulation intensity, which must be adjusted each time the electrodes are reapplied.
(ii) Electrical stimulation can be applied at the time of surgery when the nerve is exposed but it is generally important to minimize the duration of surgical procedures to minimize post-operative complications and morbidity. This is particularly true when the surgical repair is performed with tourniquet occlusion of the blood supply to the limb or when the patient requires complex medical and surgical care to deal with multiple consequences of a traumatic injury.
(iii) A conventional electrical stimulator can be implanted into the body and an electrode can be implanted on or near the repaired nerve and connected to the stimulator by an electrical lead. However, this considerably complicates the scope of the repair surgery and risks further damage or infection of the repaired nerve and surrounding tissues, which may already be highly traumatized by the original injury.
(iv) It is possible to implant electrodes temporarily at the site of the surgical repair, bringing electrical leads out through the skin of the surgical incision or another percutaneous site for connection to a conventional electrical stimulator. However, this poses the dangers of inadvertent mechanical traction or spread of infection to the already traumatized tissues surrounding the injured and repaired nerve. It also complicates general nursing and wound care during the extended period during which treatment may be desired.
Further, A. A. Al-Majed, C. M. Neumann, E. Brustein, and T. Gordon. Brief Electrical Stimulation Promotes the Speed and Accuracy of Motor Axonal Regeneration. The Journal of Neuroscience 20 (7):2602-2608, 2000 have demonstrated that stimulation produces augmentation of motor axonal regeneration into muscle nerves when delivered at the time of the nerve repair. No system however was described for effecting this in practice.